Previous high speed multiplying digital to analog converters did not allow the input signal to have a true zero value. Therefore one could not change the gain of the amplifier without changing the DC offset. This was a particular problem when dealing with auto convergence circuits of a color CRT circuit since changes must be done in real time, e.g. the screen cannot be blanked, changed and then redisplayed. Changes are done with the three color beams in motion and the display on.
Generally, previous multiplying digital to analog converters were not fast enough for this application and they did not allow a true zero input to produce a true zero output. Therefore, since one could not change the gain without altering the D.C. offset and the desired changes could not be done fast enough, the quality of the display presented was degraded during any change in the display. This was becoming more of a problem as displays became faster.
In accordance with the preferred embodiment of the present invention, a four quadrant multiplier (similar to the Gilbert Gain Cell of U.S. Pat. No. 3,689,752) is controlled by an 8-bit digital word. When the A.C. input is zero, the gain may be changed by changing the 8-bit digital word. There is no change in the D.C. offset of the output since a true zero input produces a true zero output in the preferred embodiment.
A differential input voltage is converted into differential currents. The differential currents are input to a high speed four quadrant multiplier which provides differential output signals. The gain of the multiplier and the DC offset are each independently controlled by a digital to analog converter in response to an eight bit digital word and a reference current. The preferred embodiment has improved speed of the analog throughput since the analog throughput speed is independent of the speed of the digital to analog converter.